2000 Cadillac Deville, Fuel Gauge Reads Full All Of The Time

Customer stated that fuel gauge always reads full but the trip computer is keeping track of fuel mileage and he has been using it to monitor fuel level.

Checked codes and found codes stored for both fuel level signal voltage high and low. Consulted repair information and found little help. Wiring diagram shows signals from tank go into PZM (also known as BCM), PZM sends information to instrument cluster. Could not find any info on testing so I decided to go with experience and started at the tank sending unit.

Well I was working alone and could not see the gauge while testing so I decided to test at the PZM. It is located behind the back seat and I could watch the gauge through the back glass. I disconnected the PZM and tested the tank sending unit resistance at the brown and violet wires. Open. Well, I know the tank sending unit is faulty but is the rest of the system okay?

I installed my trusty decade resistance box and the gauge reacted properly. Here are the resistance specs I was able to establish.

215 Ohms Full
167 Ohms 15 Gallons
121 Ohms 10 Gallons
78 Ohms 5 Gallons
54 Ohms 3 Gallons

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